Operators of gas turbine engines used in generating electricity at powerplants desire to have the most efficient operations possible in order to maximize their profitability and limit the amount of emissions produced and excess heat lost. In addition to maintenance costs, one of the highest costs associated with operating a gas turbine at a powerplant, is the cost of the fuel burned in the gas turbine, either gas, liquid, or coal. Increasing the efficiency of the gas turbine will result in an increase in electrical generation capacity for a given amount of fuel burned. Alternatively, if additional electrical generation is not possible or desired, the required level of electricity can be generated at a lower fuel consumption rate. Under either scenario the powerplant operator achieves a significant cost savings while simultaneously increasing the powerplant efficiency.
A significant way to increase the gas turbine engine performance is to provide the turbine with a higher supply pressure from the combustor. For a combustion system having a known pressure loss, this can be accomplished by reducing the pressure losses to the air that occurs in the region between the compressor outlet and the combustion chamber. One specific component in this region is the transition duct, which connects the combustion chamber to the turbine inlet, thereby transferring the hot combustion gases to the turbine. These gases can often times reach temperatures upwards of 3000 degrees Fahrenheit. Therefore, in order to provide a transition duct capable of extended exposure to these elevated temperatures, careful attention must be paid to the cooling of the transition duct. Often times cooling air is not used in the most efficient manner with regards to limiting the amount of pressure loss that occurs when cooling the transition duct. As a result an unnecessary drop in supply pressure to the turbine occurs, yielding a lower turbine efficiency and engine performance.
Referring to FIG. 1, a transition duct 10 of the prior art is shown in partial cross section. Transition duct 10 comprises an inner wall 11, an impingement sleeve 12, thereby forming a cooling channel 13 therebetween. Impingement sleeve 12 includes a plurality of cooling holes 14 that allow cooling air, which is indicated by the arrows, to enter cooling channel 13 and impinge along inner wall 11 to cool the transition duct. Directing a large plenum of air through cooling holes 14 causes a substantial pressure drop to occur in the air flow. It has been estimated, that for the gas turbine in which transition duct 10 is designed to operate, approximately 1.5% of the total air supply pressure from the compressor is lost due to the geometry of impingement sleeve 12 including cooling holes 14. Utilizing an alternate cooling configuration for transition duct 10 can recover a majority of this pressure loss.
The present invention seeks to overcome the shortfalls of the prior art by providing a transition duct that utilizes an improved cooling configuration that has a substantially lower pressure loss than that of the prior art.